The present invention relates to a magnetic resonance imaging (hereinafter referred to as an “MRI”) apparatus, and more particularly, to a structure of the MRI apparatus for suppressing variations of a static magnetic field and noise of the apparatus.
In the MRI apparatus, a pulse electric current is applied to a gradient magnetic field coil (hereinafter referred to as a “GC”) to generate a gradient magnetic field. Since the GC is located in a static magnetic field, Lorentz force acts on the conductor of the GC where the pulse electric current flows, thereby GC vibration is caused. The GC vibration reaches through a support structural member and the like to a static magnetic-field generation source to vibrate the static magnetic-field generation source. The generated micron-order vibration of the static magnetic-field generation source varies the static magnetic field with time, and as a result, adversely affects an MR image. Further, the GC vibration causes aerial vibration, i.e., noise, to increase a patient's feeling of anxiety, and further, disturbs communication between the patient and an operator. Patent Document 1 (Japanese Patent Laid-Open No. 2003-220050) discloses an example of MRI apparatus having a structure for suppressing the GC vibration. According to the structure, a coil container is reinforced such that the rigidity of the coil vessel is increased, thereby the vibration of the coil and magnetic poles can be reduced. Further, as another example, in an apparatus described in Patent Document 2 (Japanese Patent Laid-Open No. 2002-85371), the GC, which is the vibration generation source, is accommodated in a vacuum vessel, and a vibration damping member is used as the GC support member to suppressing the vibration.
In the structure of the MRI apparatus shown in the Patent Document 1, as the coil vessel is reinforced, the vibration caused therein can be suppressed up to a point but the vibration caused in the vacuum container cannot be suppressed without difficulty.
In the superconductive magnet apparatus shown in the Patent Document 2, as the air-propagating vibration is reduced by evacuation around the GC and a damping mechanism is utilized in the GC support member, the reduction of vibration and noise can be realized. However, in the structure, the vibration from the GC is transmitted via the GC support member to the outside, and at last, the vibration from the GC cannot be suppressed without difficulty.
In comparison with these conventional techniques, the present invention has an object to provide an MRI apparatus where variation of static magnetic field and noise due to GC vibration can be suppressed sufficiently.